When a steel vessel is used for containment, a separate concrete structure is required to provide radiation shielding. This separate structure therefore cannot provide resistance to internal pressure, but must carry inertial seismic loads caused by the earthquake.
When a lined concrete structure is used for containment, the liner, which is usually steel, has no significant strength to resist internal pressure loads, and has not been considered to be effective in resisting membrane shear stresses. The concrete structure is usually designed to resist all significant applied loads.
For the composite nuclear containment structure, a concrete structure is placed in intimate contact with, and is connected to a steel vessel. The composite structure may be proportioned so that both the concrete structure and steel vessels act together in resisting all applied loads. Since both essential components of the composite nuclear containment structure act, each structure need not have the strength required of an independent structure. This provides for a design for which the thickness of the steel vessel can be kept below that thickness requiring complex welding procedures and the concrete structure need not have the quantity of reinforcing bars or prestrung tendons required for the usual concrete containment structure.
In addition to the enhanced strength of a composite nuclear containment structure with lesser quantities of materials than usually required, construction procedures may be employed which are much less costly and time consuming than those previously required for either the steel vessel or the lined concrete structure. The composite nuclear containment may be constructed such that the steel vessel provides support for an interior crane prior to concrete placement, and provides support for winches which are used to place reinforcing bars and tendons. These winches can be used for slip forming during concrete placement and the steel vessel acts as an interior concrete form without the need for extensive temporary or permanent stiffening.
This invention relates, in general, to nuclear energy containment systems, and more particularly, to a combination of structural materials which reduce quantities of materials or stresses or both, and will permit techniques which will facilitate construction.
Nuclear steam generating systems are contained within a containment structural system, which must be capable of resisting internal pressure and temperature increases caused by the Design Basis Accident (effects of an assumed loss of coolant accident) in combination with other loading requirements. The system must also be capable of providing radiation shielding during the entire life of the nuclear steam generating system, including the Design Basis Accident. These requirements have been met heretofore in several ways. One method is the application of steel containment supported by a foundation mat which serves to provide leak tightness and resist the internal pressure. A concrete structure, separated from the steel vessel, usually surrounds the steel vessel in order to provide radiation shielding and protection for the steel vessel against the effects of tornadoes and other external loadings. This outer concrete structure is usually supported on the same foundation mat as the steel vessel. During the Design Basis Accident, the steel vessel expands as it is subjected to internal pressure and temperature increases. Because of the space provided between the steel vessel and outer concrete structure is not significantly affected by the Design Basis Accident. However, both structures must be capable of withstanding seismic loads which may be assumed to be coincident with Design Basis Accidents. Another method consists of a concrete structure which provides the primary structural resistance to all imposed loads. The interior of the concrete structure is lined with a membrane, usually metallic, in order to provide resistance to leakage. During a Design Basis Accident, pressure load is passed to the concrete structure which is usually reinforced with steel bars, or prestressed by means of tendons, or a combination of both. As the liner is heated during the Design Basis Accident, it is constrained by the structural concrete which carries the pressure load. Because of this constraint, the liner, especially a metallic liner, will develop either low tensile mechanical strains, or in fact, compressive mechanical strains. The development of compressive mechanical strains during the Design Basis Accident is particularly evident in prestressed concrete containments. The development of high compressive mechanical strains, and hence compressive forces in the liner will be balanced by corresponding tensile forces in the surrounding concrete structure. Therefore, the liner, while providing leakage resistance, may impose additional loads upon the concrete structure containing embedded reinforcing bars and/or prestressing tendons. For reasons of economics, the liners have been selected as thin as practical consistent with requirements of construction. In most applications, the liners range from about 1/4" to 1/2" in thickness.
U.S. Pat. No. 3,605,362 issued Sept. 20, 1971 to E. A. Sweeney and assigned to Stone and Webster Engineering Corporation, describes structural assemblies which reduces discontinuity stresses, and was considered to be especially suitable for reinforced concrete nuclear power reactor containment wall connecting to the supporting mat. An inside liner covers the interior of the walls and dome as well as the top of the supporting mat.
U.S. Pat. No. 3,444,725 issued on May 20, 1969 to C. T. Chave and assigned to Stone and Webster Engineering Corporation, describes a nuclear containment system consisting of an inner and outer containment structure in which the annular space between the two structures is used for a leak detection system.
U.S. Pat. No. 3,725,198 issued on Apr. 3, 1973 to G. A. Harstead, et al, and assigned to Westinghouse Electric Corporation, discloses a system which consists of an inner and outer structure supported on a common foundation mat, the space between the two structures being filled with a fluid.
While the aforementioned containment systems satisfy the requirements of containment, they usually require either a dual structural system or a concrete structure with a metal liner which contributes, at best, little to the overall structural resistance capability of the nuclear containment system.
Each structure of the dual system must be capable of withstanding independently the considerable loads to which it may be subjected. On the other hand, the metal lined concrete structure will be subjected to high stresses at discontinuities caused by internal pressure, and the previously described thermal effects of the liner, during the Design Basis Accident in addition to the general loading imposed by internal pressure thermal effects, seismic inertial loads, tornado effect, etc., the metal liner offering little to structural capability.